1. Field of the Invention
The present embodiments relate to organic metal compounds in which compounds for host and compounds for dopant are connected, organic electroluminescence display devices using the compounds and a method for preparation of the devices. More precisely, the present embodiments relate to organic metal compounds in which compounds for host and compounds for dopant are connected to make energy transmission between host and dopant possible on a molecular level, organic electroluminescence display devices using the same and a preparation method thereof.
2. Description of the Related Art
As a self-emissive display device, electroluminescence display devices (EL devices) have a broad visual angle, excellent contrast, and a quick response time.
EL devices are divided into inorganic EL devices and organic EL devices according to the emitting layer forming materials. Organic EL devices have advantages over inorganic EL devices such as excellent brightness and driving voltage, a quick response time, and polychromy.
In general, in the structure of an organic EL device comprises anodes formed on a substrate, and in the upper part of the anode, a hole transport layer, an emitting layer, an electron transport layer and a cathode are formed in that order. The hole transport layer, emitting layer, and electron transport layer are organic thin films composed of organic compounds.
The drive principal of organic EL devices having the above structure is as follows.
Once voltage is applied between the anode and the cathode, holes are infused from the anode into the emitting layer via a hole transport layer. In the meantime, electrons are infused into the emitting layer from a cathode via an electron transport layer. In the region of the emitting layer, carriers are rearranged to form exitons. The excited exiton is transformed into ground state, resulting in emission of the emitting layer molecules. As a result, images are formed. Emitting materials are classified according to emitting mechanism into two groups; one is composed of fluorescent materials using exitons in the state of singlet, and the other group is composed of phosphorescent substances using exitons in the state of triplet.
Phosphorescent substances have an organomineral compound structure containing generally heavy atoms, by which an exiton can be transformed from the state of triplet, a forbidden transition, through allowed transition. Phosphorescent substances have much higher emitting efficiency by using triplet exiton, having 75% generation ratio, than fluorescent materials which use singlet exiton with 25% generation ratio.
An emitting layer formed by phosphorescent substances is composed of a host material and a dopant material which is luminous by energy transmission from the host material. Dopant materials include various iridium metal compounds. As a part of study on organic electroluminescent materials using iridium compounds, research teams at Princeton University and University of Southern California reported phosphorescent substances based on iridium, and platinum metal compounds. But more studies are ongoing to develop a better luminous stable material.
While a low molecular weight organic EL material must be formed by using a dry process such as vacuum deposition, a high molecular weight EL material can be formed by using a wet process such as spin coating, etc. The low molecular weight EL material cannot be formed by a wet process because of its low solubility. Although the high molecular weight EL material has a solubility high enough to form a device by wet process, it has a low emission property, in particular, a shorter lifetime than the low molecular weight EL material. Thus, it is required to develop an organic EL material having high solubility to form a device by wet process, an easier and more economical process than deposition, and at the same time having emission properties as high as a low molecular EL material.